Myosin light chain kinase (MLCK) is a key element in the regulation of smooth muscle contraction. Phosphorylation of MLCK, in particular, has emerged as an event that may link contractility to other signaling pathways. In this application we aim to fill the gaps in our knowledge of how site-specific phosphorylation affects the MLCK activity. Our long term goal is to understand the signal transduction process in smooth muscle cells and the mechanisms for its regulation. Specifically, we propose (i) to develop phospho-specific antibodies to monitor the phosphorylation level of MLCK in cells and to assess the relationship between phosphorylation of MLCK and the contractile properties of smooth/non-muscle cells; (ii) to test the hypothesis that phosphorylation of MLCK by MAPK affects the activity of the enzyme and to explore the mechanism by investigating effects of MLCK phosphorylation on its conformation; and (iii) to determine how phosphorylation by MAPK and PKA affects interaction of MLCK with components of smooth muscle and non-muscle contractile machinery by elucidating the in situ localization. The proposed research is a logical extension of the ongoing Program Project Grant funded by NIH (P01-AR41637) under the title of "Molecular Mechanisms of Smooth Muscle Regulation." The proposed collaboration will not only enhance the research program of the applicant (C.-L. Albert Wang, Ph.D., the US Principal Investigator), but will also benefit the scientific interests of the Foreign Collaborator, Dr. Alexander Vorotnikov, and intensely impact the research conducted in the Foreign Laboratory. Work described in this proposal will provide information to extend the applicability of the ongoing studies to the regulation of MLCK by phosphorylation, and afford a better understanding of smooth muscle regulation.